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Digital pulse position modulation (PPM) is a transmission format that can be used to exchange excess channel bandwidth for improved receiver sensitivity. This thesis is concerned with the performance of digital PPM for use in optical fibre systems such that increased single-span transmission distances can be achieved. It includes a literature survey and a thorough theoretical investigation into uncoded directly detected and coherently detected digital PPM, along with an evaluation of forward error correction coded PPM. Original contributions are described for digital PPM systems employing PIN-BJT transimpedance and PIN -FET high impedance pre-amplifiers. In the PIN -BJT case, a new analysis is presented that accounts for the band-limiting effect of the preamplifier and the non-white noise power spectral density, resulting from equalisation. For the PIN-FET, a causal noise-whitening filter is considered and the factors that limit the maximum bit-rate determined. By using an upper Chernoff Bound to the average binary error probability, results are calculated for APD-BJT and APD-FET PPM systems. The practical implementation of the optimal PPM filter is considered and two novel techniques are described for automating the realisation process. Coherent PPM is investigated and new work is presented for Reed-Solomon coded homodyne PPM. The influence of the Reed-Solomon code rate on receiver sensitivity is studied and it is shown that a 3/4 code rate leads to optimum sensitivity. Digital PPM employing soliton pulses is examined and it is shown that, under ideal conditions, a higher bit-rate than PCM can be achieved. The influence of timing jitter, associated with soliton pulse energy fluctuations and the Gordon-Haus effect, on the performance of digital PPM is investigated. The simplification of the PPM pre-detection filter is studied and a range of suboptimum filters investigated. The calculations demonstrate that the optimal predetection filter can be replaced by a simple 3rd order filter without incurring a severe sensitivity penalty. The sensitivity results of the various digital PPM systems investigated in this thesis are compared to those of equivalent PCM systems. It is illustrated that digital PPM offers an improvement of between 5-10 dB depending upon which coding format/ detection technique is employed. This represents an increase in transmission distance of 25-50 km and demonstrates that digital PPM is a promising modulation format and has potential for future high performance telecommunication routes.

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Modern fiber-optic networks leverage massive parallelization of communications channels in the spectral domain, as well as low-noise recovery of optical signals, to achieve high rates of information transfer. However, quantum information imposes additional constraints on optical transport networks – the no-cloning theorem forbids use of signal regeneration and many network protocols are premised on operations like Bell state measurements that prize spectral indistinguishability. Consequently, a key challenge for quantum networks is identifying a path to high-rate and high-fidelity quantum state transport.<div><br></div><div>To bridge this gap between the capabilities of classical and quantum networks, we developed techniques that harness spectral multiplexing of quantum channels, as well as that support frequency encoding. In relation to the former, we demonstrated reconfigurable connectivity over arbitrary subgraphs in a multi-user quantum network. In particular, through flexible provisioning of the pair source bandwidth, we adjusted the rate at which entanglement was distributed over any user-to-user link. To facilitate networking protocols compatible with both spectral multiplexing and frequency encoding, we synthesized a Bell state analyzer based on mixing outcomes that populate different spectral modes, in contrast to conventional approaches that are based on mixing outcomes that populate different spatial paths. This advance breaks the tradeoff between the fidelity of remote entanglement and the spectral distinguishability of photons participating in a joint measurement.<br></div><div><br></div><div>Finally, we take steps toward field deployment by developing photonic integrated circuits to migrate the aforementioned functionality to a chip-scale platform while also achieving the low loss transmission and high-fidelity operation needed for practical quantum networks.<br></div>